Impact of energy status on the serotonergic regulation of energy balance, the rapid escalation of obesity rates in Americans, combined with the resistance of this condition to current treatment approaches, highlights the need for new insights into neurobehavioral mechanisms regulating energy balance. Although treatments employing pharmacological manipulation of the brain serotonin system have demonstrated efficacy, the neurobehavioral mechanisms through which serotonin modulates food intake and energy expenditure remain unclear. Of the many known serotonin receptor subtypes, 5-HT1B and 5- HT2C receptors have been most strongly implicated in the serotonergic suppression of food intake. We have found that null mutations of genes encoding these 5-HT receptor subtypes (htr1b- and htr2c-) influence feeding differently in ad libitum fed animals vs. animals that had been fasted. These and additional preliminary data reveal that the energy status of an animal markedly influences the manner in which the serotonin system regulates energy balance. The elucidation of neural mechanisms underlying energy status-dependent serotonergic regulation of energy balance could facilitate the development of novel pharmacotherapeutic approaches to obesity. In this proposal we test the hypothesis that energy status-dependent serotonergic regulation of energy balance is mediated through pathways involving hypothalamic arcuate nucleus neurons that express 5-HT1B and 5-HT2C receptors. In Aim 1 we will test the hypothesis that energy status-dependent effects of the htr1b- and htr2c- mutations on food intake are associated with energy status-dependent influences on the physiological and behavioral determinants of energy balance. Both global and cell type-specific mutations of these receptor genes will be performed with a particular focus on receptor subpopulations expressed in the hypothalamus. In Aim 2 we will test the hypothesis that energy status-dependent effects of htr1b- and htr2c- mutations on the physiological and behavioral determinants of energy balance are mediated by pathways involving hypothalamic arcuate nucleus neurons. Toward this end, we will examine patterns of hypothalamic neuropeptide gene expression and patterns of neuronal activation induced by fasting. In Aim 3 will generate and validate conditional mutant mice to selectively eliminate 5-HT1B and 5-HT2C receptor expression in neurons expressing the neuropeptides NPY/AGRP and POMC/CART, respectively.